In recent years thermal imaging technology has developed a capability of providing images of television quality or better for various applications, such as aerial terrain mapping, target determination and acquisition, surveillance, electrical fault location, medical imaging, and irrigation control.
One particularly useful technique for thermal imaging is known as "cool IR". This technique has the advantage of being able to carry out imaging over great distances, in total darkness, on camouflaged objects and through cloud cover. Cool IR systems require an IR detector to be cooled to the temperature of liquid air, about 77 K, for efficient operation.
Various types of cryogenic refrigerators are known for cool IR applications. These include liquid nitrogen cryostats, Joule-Thomson coolers and closed cycle cryocoolers. For certain applications, closed cycle cryocoolers are preferred.
There exist a variety of configurations of closed cycle cryocoolers. These include Stirling, Vuilleumier (VM) and Gifford-McMahon (GM) cryocoolers. A preferred configuration is the integral type.
A basic integral Shirling crycooler comprises a compressor section and an expander-displacer section combined in one integrated package. Reciprocating elments of both the expander-displacer and the compressor are mechanically driven via a common crankshaft. The integral configuration guarantees a prescribed displacer stroke and displacer/compressor phase relationship, but it involves a disadvantage in that the vibration output of the compressor is transmitted to the cooled device due to the close proximity of the components.
A further disadvantage in integral Stirling cryocoolers lies in their compressor seals. Various types of dynamic compressor seals are employed, including clearance seals. These tend to wear overtime, releasing particulate matter into the system; this interferes with the operation of the Stirling regenerator.
Additional contamination of the regenerator is caused by lubrication materials and other materials associated with parts of the drive motor which are generally located in fluid communication with the regenerator.
An integral Stirling cryocooler which overcomes the above-described disadvantages is described in unpublished copending Israel Patent Application No. 78933 filed May 26, 1986.
Split Stirling cryocoolers are also known in the prior art. Split Stirling cryocoolers overcome the problem of transmission of vibrations to the cooled device, encountered in integral cryocoolers. However, in view of the fact that the displacer of a split cryocooler is not mechanically connected to the motor, problems of nonuniformity of displacer motion occur. These problems arise from instability of the pressure of the pulses produced by the compressor due to use of a dynamic seal and instability on the applied damping force.
One example of a split Stirling cryocooler is a cryocooler manufactured by Ricor in Israel having apparatus for producing a magnetic damping force. This apparatus has the disadvantage that electromagnetic fields are generated thereby, causing possible interference with sensitive electrical and electro-optical apparatus in the vicinity thereof and thus requiring extensive shielding. Additionally, the magnetic damping is extremely difficult to fine tune to provide optimized damping. The above Ricoh cryocooler is described in U.S. Pat. No. 4,514,987, which shows the use of a viscous friction damper wherein a narrow circumferential gas flow passage is defined between a piston and a cylinder in which the piston moves.
Another type of split Stirling cryocooler employs a dynamic seal. Cryocoolers of this type are manufactured by Martin Marietta and CTI in the U.S.A. and have the disadvantages described hereinabove in connection with compressor seals.